1. Field of the Invention
The present invention relates to a toner to be used for electrophotography, an image formation method for visualizing an electrostatic charge image, and a toner jet method.
2. Description of the Related Art
As general electrophotography, there has been known a method involving forming a latent image on an image bearing member (photosensitive member), visualizing the latent image by supplying a toner to the latent image, transferring the toner image onto a transfer member such as paper, and fixing the toner image onto the transfer member with heat or pressure to obtain a duplicate.
In order to save power and shorten wait time in an electrophotographic apparatus, an on-demand type fixing device has been put into practical use as a fixing device, the on-demand type fixing device including a combination of a ceramic heater having small heat capacity and a film.
In the above-mentioned fixing device, an attempt has been made to reduce an internal pressure of a fixing nip of the fixing device from the viewpoints of prolonging life and handling various media.
Further, along with the recent increase in printing speed, the time during which a toner and a medium such as paper pass through a nip of the fixing device is becoming short year after year.
Further, there is a situation in which the output onto thin paper by double-sided printing is increasing owing to the users' enhanced awareness of energy saving.
Further, in recent years, there are increasing chances that users output graphic images each having a high printing ratio, such as image data and posters captured by digital cameras, mobile terminals, and the like, through use of an image forming apparatus such as a laser printer (LBP).
Against such a background, there has been a demand for a toner which can be used for double-sided image formation using thin paper even at a high image formation speed under a low pressure of a fixing nip.
In order to satisfy the demand, it is necessary to achieve low-temperature fixability of a toner which is superior to the conventional one. As a technology for improving the low-temperature fixability, there have been a great number of proposals concerning using a crystalline resin as well as an amorphous resin as a binder resin.
It is known that the crystalline resin can improve the low-temperature fixability by being rapidly melted in the vicinity of its glass transition temperature to enhance the compatibility with the amorphous resin (for example, Japanese Patent Application Laid-Open No. 2010-102058).
Further, in order to improve the low-temperature fixability and enhance the effect of dispersing a release agent through use of the crystalline resin, there has been proposed a toner containing a graft copolymer of a polyolefin resin and a vinyl copolymer (for example, Japanese Patent Application Laid-Open No. 2011-138120).
However, when the compatibility between the crystalline resin and the amorphous resin is too high, there arises a problem in that the heat-resistance storage property and crystallinity of a toner are rather degraded.
In contrast, when the compatibility between the amorphous resin and the crystalline resin is degraded, although a crystal of the crystalline resin tends to be formed more easily, the amorphous resin and the crystalline resin are difficult to be compatible with each other even at a melting point or more. Therefore, it is difficult to improve the low-temperature fixability, in particular, in the case of short fixing time or a low internal pressure of a nip.
In order to solve the above-mentioned problem, there has been disclosed a technology for accelerating recrystallization of a crystalline resin by adding the step of performing heat treatment at particular temperature during the production of a toner (Japanese Patent Application Laid-Open No. 2010-152102).
Certainly, with the technology, a toner can be obtained which contains a crystalline resin having a high degree of crystallinity and which is excellent in heat-resistance storage property.
However, once the toner is melted during a fixing step, the crystalline resin and the amorphous resin become compatible with each other and do not return to a phase-separated state formed of the crystalline resin and the amorphous resin even when cooled. Therefore, when transfer paper on which a fixed toner image has been formed is left to stand in an overlapped state or in contact with another member under a high-temperature and high-humidity environment, the toner image adheres to the transfer paper or the member with which the toner image comes into contact in some cases.
Meanwhile, there has been proposed a procedure for accelerating the crystallization of a crystalline resin by adding a crystal nucleating agent to a toner.
As the crystal nucleating agent, there have been proposed an inorganic crystal nucleating agent of a fine particle such as silica (for example, Japanese Patent Application Laid-Open No. 2007-033773) and an organic crystal nucleating agent such as a benzoic acid metal salt or fatty acid amide (for example, Japanese Patent Application Laid-Open No. 2006-113473).
However, when the formulated amount of the inorganic crystal nucleating agent such as silica is large, the inorganic crystal nucleating agent exhibits a filler effect to enhance the melt viscosity of a toner, which may inhibit the low-temperature fixability. Further, the inorganic crystal nucleating agent influences the charging characteristics in most cases, with the result that it becomes difficult to control the chargeability in most cases.
On the other hand, the organic nucleating agent is a low-molecular compound such as a benzoic acid metal salt or a fatty acid metal salt in most cases. Such a crystal nucleating agent is likely to be segregated on the surface of a toner. As a result, the effect of the crystal nucleating agent becomes insufficient, and the storage property of the toner is degraded and the storage stability of a toner image is degraded in some cases. Further, after the toner is fixed, parts of crystals are in an incomplete crystalline state even when being recrystallized. Therefore, when the fixed toner is left to stand for a long period of time, a crystalline part and a compatible part are phase-separated, and a fixed image is curled in some cases owing to the reduction in volume of the crystalline part.
Accordingly, there remain a great number of technical problems to be solved for satisfying both the low-temperature fixability and the long-term storage stability of a fixed image, and the conventional toner still has room for improvement.